Method for providing temperature compensation for a wheatstone bridge-type pressure sensor

ABSTRACT

A method of compensating for differences of temperature, in a pressure sensor of the kind wherein a pressure is sensed by use of a diaphragm connected in a Wheatstone bridge circuit. First measurement points of offset output signals emanating from the bridge in the pressure sensor at a number of temperature levels are plotted in a first graph indicating the voltage as a function of the resulting resistance of the bridge, whereupon adjacent measurement points are interconnected by straight lines and calculation is effected through interpolation between the measurement points. The sensitivity of the pressure sensor such as voltage/pressure unit, is determined in a corresponding manner at different temperature levels by plotting second measurement points in a second graph indicating the sensitivity as a function of the resulting resistance in a similar way the last mentioned measurement points are interconnected by straight lines, and calculation is effected through interpolation between the measurement points. The resulting curves are processed in a computer, whereupon a signal from the pressure reading provides a directly temperature-compensated pressure value which may be read from a third graph indicative of an output signal that is proportional to the prevailing pressure.

BACKGROUND OF THE INVENTION

The invention concerns a method for compensation of temperature in apressure sensor of the kind designed to convert a detected pressure intoan electric signal which is registered by a measuring device.

A prior-art method used for this purpose consists of applying a numberof resistances on a pressure-sensitive diaphragm, which resistanceschange their resistance values in response to changes in the diaphragmconfiguration when the diaphragm is exposed to pressure changes. As arule, the resistances are electrically connected in a bridge circuit, ofthe kind known as a Wheatstone bridge.

All transducers, independently of whether they comprise resistancesmanufactured from thin films or thick films on an insulating material orthrough ion implantation in silica materials (known as piezoresistivetransducers), suffer from the disadvantage of being extremelytemperature-dependent. This means that the transducer output signalwhich is representative of a pressure at a certain moment, changes asthe temperature changes, also when the pressure remains constant. Thistemperature dependency could, in some cases, be extremely large.

When a pressure transducer is used in applications where the temperaturevariations are comparatively small, it may be calibrated once theenvironmental conditions have stabilized somewhat. Thereafter, theeffects of temperature changes are essentially negligible.

Other prior-art methods of temperature compensation consist ofconnecting resistances having known temperature charactersistics indifferent diagonals of the bridge, or of actively compensating theoutput signal for temperature influence by means of electronics builtinto the transducer. However, in this case also, the electronics areexposed to the temperature variations and consequently these variationsneed again be considered. As long as the temperature variations remainsmall it is, however, possible to obtain acceptable measurement resultsby any one of the above methods.

In thermal sterilization by means of vapor in an autoclave theconditions are, on the other hand, very difficult to master becauseduring one sequence the pressure varies from between approximately 30mbar absolute pressure and approximately 5 bar absolute pressure, i.e. apressure ratio of approximately 1:150. At the same time, the temperaturevaries between approximately 20° C. and 140° C. In addition, thechangeovers between maximum and minimum values and vice versa, bothrespect to pressure and temperature, take place rapidly, and severaltimes, over a comparitively short period, during one process.

However, the above-mentioned prior-art stabilizing methods are quiteinsufficient in this application to meet the requirements as to accuracywith respect to the precision of the measured values in sterilization.Particularly in the case of low pressures the percentage deviations ofmeasured values are quite considerable, often several hundred percent.At the same time, low pressures are an important measuring range in thisconnection.

For such an extreme activity as vapor sterilization various measurestherefore have been taken to prevent the effects of temperature on thepressure sensor. Examples of such measures are isolation of the pressuresensor from the autoclave enclosure where the pressure is measured,transfer of the pressure to the sensor by way of, for instance, an oilbuffer, a water bag or the use of capillary tubes. Another way is tocool the pressure sensor to prevent the pressure sensor from followingthe temperature increase inside the autoclave enclosure.

However, all these various previously known methods of preventing atemperature rise in the pressure sensor cause cavities to form betweenthe autoclave chamber and the sensor. Such cavities are not desired, andnormally they are not accepted by established standard specifications,because inside them an environment is created that favors collection andgrowth of micro-organisms. Instead, a desired end is to be able toposition the sensing diaphragm of the pressure sensor directly in thewall of the autoclave enclosure, without using any intermediary tubingsor pipings.

Of course, it is always possible to position a separate temperaturesensor on or adjacent the diaphragm for the purpose of measuring theexisting temperature during the pressure sensing operation. However,this does not solve the problem, since the pressure sensor only providesinformation on the temperature at its place of mounting on the diaphragmand not on the actual effects to which the pressure-sensitiveresistances are exposed.

SUMMARY OF THE INVENTION

The subject invention provides a method making it possible to positionthe diaphragm of the pressure sensor, as desired, directly in the wallof an autoclave chamber, in order to eliminate undesired cavities whileat the same time temperature compensation is achieved in all temperatureranges that are relevant in the autoclave process, providing excellentmeasurement results within the measurement range in question. The typeof pressure sensor that may be considered in this connection is onewhich detects a pressure by means of a diaphragm which is connected inan electric bridge circuit consisting of four resistances interconnectedin series in a quadrangle, known as a Wheatstone bridge, thoseresistances changing the values in response to changes of configurationof the diaphragm caused by changes in the detected pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be defined in closer detail in the following withreference to the accompanying drawings wherein

FIG. 1 illustrates a bridge circuit which serves as the measuring meansin a pressure sensor, not illustrated, in an autoclave and which isconnected to the diaphragm of the pressure sensor,

FIG. 2 is a graphical representation wherein the slope of the curvesillustrate the sensitivity of the pressure sensor expressed in mV/bar,

FIG. 3 is a graphical representation wherein the curves represent thesensitivity of two different pressure sensors as the function of theresulting resistance across the bridge circuit, and

FIG. 4 is a graphical representation wherein curves represent a socalled offset with respect to two different pressure sensors as afunction of the resulting resistance across the bridge circuit.

DETAILED DESCRIPTION

The method in accordance with the invention is intended to be utilizedwith the type of pressure sensor wherein the sensor is provided with adiaphragm which alters its configuration in response to pressure changesand which is connected with an electrical bridge circuit 1. As appearsfrom FIG. 1, this bridge circuit consists of four interconnectedresistances 2, 3, 4, and 5 which are connected in series in quadrangleand form a circuit generally referred to as a Wheatstone bridge. Theresistances 2, 3 ,4, 5 change their values primarily in response tochanges of the configuration of the diaphragm, but are affected also bychanges in temperature. In order to obtain an electrical measurementsignal in two diagonally opposite points 6, 7 in the bridge circuit 1when measuring a pressure, two oppositely positioned points 8, 9 in thebridge circuit 1 are supplied with current or voltage. This supply ofcurrent or voltage is constant, in order not to affect the output inpoints 6, 7.

When the supply is in the form of a constant voltage, the voltage dropbetween the two supply points 8, 9 will always be constant,independently of how the four resistances 2, 3, 4, 5 change their valuesindividually. The output signal between the two other points 6, 7follows the values of the four resistances in the conventional manner.

However, if the bridge 1 is supplied with constant current, it becomespossible to also measure the total resistance of the bridge across thetwo supply points 8, 9. This magnitude is an excellent reference of thetemperature dependency of the pressure sensor, since it is directlyrelated to the resistances 2, 3, 4, 5 that are active for pressurereadings.

Each pressure sensor has a sensitivity expressed, for instance inmV/bar. In the ideal case, the output signal from a pressure sensor, asillustrated in FIG. 2, forms a straight line departing from zero andwith a slope corresponding to the sensitivity of the pressure sensor. Inreality, the zero point (offset) as well as the slope (sensitivity) areaffected by the temperature.

In the manufacture of pressure sensors, the offset and slope aremeasured as functions of the total bridge resistance of the associatedpressure sensor across the temperature range relevant to the pressuresensor. The bridge resistance R_(B) is the relation between the voltageU_(B) across the bridge 1 and the supply current i_(k) through thebridge. The table below relates to a certain pressure sensor thetemperature dependency of which is measured from room temperature atintervals of 20° C. up to approximately 140° C. The values correspondingto the offset and the sensitivity are registered as functions of thebridge resistance at seven different temperature levels in accordancewith the example given. The measured values are stored in the memory ofa computer and are the data on which the temperature compensationcalculations are based.

    ______________________________________    R.sub.B       Sensitivity                           Offset    ______________________________________    3164          90.70    5.11    3285          90.73    5.14    3407          91.08    5.02    3570          91.50    4.83    3744          92.25    4.59    3900          93.40    4.40    4152          94.42    4.57    ______________________________________

In FIG. 3 is shown an example of a graphical representation wherein acouple of curves illustrate the measured values of the sensitivity of acouple of pressure sensors as a function of R_(B). In FIG. 4 is shown ina corresponding manner a graphical representation of the offset in acouple of pressure sensors as a function R_(B). The intervals betweenadjacent measurement points are interconnected by means of straightlines and the calculation is affected thereafter by interpolationbetween the break points, which is a simplification of a continuousfunction.

By treating each individual pressure sensor separately in this way, andfeeding in its characteristic values into a computer full, temperaturecompensation is obtained at each temperature level as a result of thecomputer calculation. In FIG. 2, dotted lines illustrate how, in thecase of an output signal, a pressure value P₁ is obtained in the subjectcase, whereas after the temperature compensation the more correct valueP₂ is obtained.

In FIG. 1, two resistances R₁ and R₂ are indicated, these resistancesbeing connected in parallel across reistances 3 and 4, respectively, andbeing inserted for the purpose of coarse temperature compensation. Inaddition, a couple of resistances R₃ and R₄ are connected, the purposeof which is to minimize the offset. Advantageously, these resistancesR₁, R₂, R₃, R₄ can, however, be positioned in the control system, wherethey are exposed to smaller temperature variations than they are whenpositioned closely adjacent a pressure sensor. This is illustrated bydash and dot lines, and by a line discontinuity 10 and 11, respectively.The coarse compensation obtained by means of these resistances R₁, R₂,R₃, R₄ requires less correction of the computer software whichconsiderably increases the measurement accuracy.

I claim:
 1. A method of providing temperature compensation for apressure sensor in which pressure is sensed by a diaphragm connected incircuit with a Wheatstone bridge having four resistances connected inseries such as to provide output valves which are related to changes inpressure exerted on the diaphragm, said method comprising the stepsof:(a) supplying the bridge at two diagonally opposite points with aconstant current while the bridge is located at a place of measurement;(b) supplying output signals from the bridge to a computer which isunaffected by temperature variations; (c) plotting on a first graph aplurality of first measurement points representing offset of thepressure sensor at a plurality of temperature levels, indicating voltageas a function of resulting resistance of said bridge; (d) on said firstgraph, interconnecting neighboring ones of said points by straightlines, and effecting calculation, through interpolation between saidpoints; (e) determining the sensitivity of the pressure sensor in termsof voltage relative to pressure, in a corresponding manner, at each of aplurality of temperature levels, by obtaining and plotting on a secondgraph a plurality of second measurement points indicating saidsensitivity as a function of the resulting resistance of said bridge;(f) on said second graph, interconnecting neighboring ones of saidsecond measurement points by straight lines, and effecting calculation,through interpolation between said second measurement points; (g)processing said first and second graphs in said computer to provide athird graph on which signals from the bridge directly providecorresponding temperature-compensated pressure valves indicative ofoutput signals proportional to respective prevailing pressures.
 2. Themethod of claim 1, further comprising:preparing said Wheatstone bridgefor use in step (a), by positioning two first resistances in parallel insaid circuit with respective ones of two of said four resistances ofsaid Wheatstone bridge, for providing coarse temperature compensation,and by positioning two second resistances in series in said circuit withrespective others of said four resistances of said Wheatstone bridge,for minimizing deviation from zero of output signals from said bridge inan environment wherein temperature variations are smaller than aretemperature variations closer to said bridge.